This invention relates generally to an electrophotographic printing system and more particularly concerns a method and apparatus which uses a set of tone reproduction curves customized to compensate for system performance changes related to two or more image attributes. For example, the invention will be described in terms of a xerographic environment where a set of tone reproduction curves is generated and used to compensate for system performance changes related to media type and halftone screen selections. However, those of skill in the art will understand that the invention can be applied to other image attribute combinations and in other image processing applications.
By way of background, digital reproduction, transfer or display of original images on image output terminals begins by creating a digital representation of an original image. Commonly, the digital representation becomes a two-tone microstructure otherwise known as a halftone or binary bitmap. In conventional halftoning, multiple gray levels or gray densities in the original image are reproduced by varying the amplitude within a fixed spatial frequency of halftone microstructures (or halftone cells/dots). Continuous tone images or image portions are typically represented in binary format by creating halftone cells or dots, where each cell represents a gray level density within an area of picture elements (pixels).
Methods of halftone digital image processing, encompassing the process steps of scanning or image acquisition through printing or display are known. In general, digital image output terminals (e.g., printers) are capable of creating spots within an area with a predetermined resolution (dots per unit area). In scanners, a single “spot” describes a multi-bit density response. Typically, input scanners may acquire density information at 256 levels of gray to describe a spot or pixel. By contrast, output terminals generally have two or another relatively small number of levels to reproduce image information.
In printing systems, maintaining stability and accuracy as to the amount of a marking material (e.g., toner or ink) being applied to a print surface is a major concern. Specifically, it is known that due to varying conditions, the amount of marking material (e.g., cyan, magenta, yellow and black toner or ink) will fluctuate from a predetermined target value. For example, humidity, toner or ink age, machine calibration, toner or ink quality can all cause the amount of marking material applied to a print surface to vary.
Therefore, for copiers and printers, a common technique for monitoring the quality of documents is to create test patches of predetermined desired densities. The actual density of the toner of the test patches can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In a xerographic environment, the surface that is typically of most interest in determining the density of printing material is a charge-retentive surface or photoreceptor, on which an electrostatic latent image is formed, and subsequently developed, by causing toner particles to adhere to areas that are charged in a particular way. In such a case, the density detector for determining the density of toner on the test patch, which is often referred to as a “densitometer,” is disposed along the path of the photoreceptor directly downstream of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system to charge or discharge, as necessary, the surface at a predetermined location.
The test patches are moved past the development unit and the toner particles within the development unit are caused to electrostatically adhere to the test patches. The denser the toner on the test patches, the darker the test patches will appear in optical testing. The developed test patches are moved past a density detector (densitometer) disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch. Xerographic test patches are traditionally printed in the inter-document zones on the photoreceptor. They are used to measure the deposition of toner to measure and adjust or update Tone Reproduction Curves (TRC). A common method of process control involves scheduling solid area, uniform halftones or background test patches. High-quality printers will often use many test patches.
Depiction of a process whereby a printing machine maintains output print density is illustrated in FIG. 1. Shown is an image path A for a print machine where a page description module (which implements a Page Description Language (PDL) such as, but not limited to, PostScript®) 10 forwards image information to raster image processor module (RIP) 12. The RIP generates a rasterized image (in this example, a 600×600×8 image) 14. Through this procedure, a contone image (e.g., having cyan, magenta, yellow and black) is described in a gray level or contone format. A Contone Rendering Module (CRM) 16 receives the rasterized image 14 and performs a halftoning operation 16a on the rasterized image 14 in accordance with a predetermined Tone Reproduction Curve (TRC) 16b. The CRM 16 then generates a rasterized binary (halftoned) image having a high addressability factor (e.g., 4800×600×1) 18. This binary image is provided to a Raster Output Scanner (ROS) 20, which in turn generates a photoreceptor image 22. Using the photoreceptor image 22, normal known xerographic operations are undertaken for the generation of a color print.
Consistent tone reproduction is a high priority in color production markets. Even slight color changes within a job can be objectionable to a customer. Within the architecture described in FIG. 1, constant tone reproduction for printed outputs over time is maintained by feeding real-time (inter-document zone) xerographic density information to the CRM 16 of the print engine. The CRM 16 applies an appropriate TRC 16b to each contone image and then the image is halftoned to a binary high-addressable image space understood by the ROS 20.
Data concerning the xerographic density of patches on photoreceptor image 22 are provided to the CRM 16 by process control feedback 24. In order to maintain a stable printing operation despite the fact that the print engine output may be varying, the tone reproduction curve (TRC) 16b is applied immediately before the halftoner operation 16a. Thus, if the signal from process control feedback 24 indicates that the xerographic density values are off a nominal amount, TRC 16b is changed in front of halftoner operation 16a in order to provide desired print outputs. For example, if the printer is determined to be printing overly high yellow amounts of toner, the tone reproduction curve will be adjusted slightly down so that the yellow toner amount requested is decreased, thereby maintaining a stable printing output. Densitometer measurements are made on a regular basis and the TRC 16b is updated on a regular basis in an attempt to maintain consistent color reproduction performance over time, temperature, humidity and other variables, so that, for example, all the prints in a print job are produced as consistently as possible, even when a customer job is not completely printed before the xerographic density state of the system changes.
FIGS. 2 and 3 illustrate an exemplary image printing system 30 which can incorporate the image path A described above or be modified to practice the methods of the present invention. Printing system 30, for purposes of explanation, is divided into image input section 32, controller section 34 and printer section 36. In the example shown, the image input section 32 has both remote and on-site image inputs, enabling system 30 to provide network, scan and print services. Other system combinations may be envisioned such as a stand-alone printing system with on-side image input (i.e., a scanner), controller and printer, a network printing system with remote input, controller and printer, among other configurations.
While a specific printing system is shown and described, the present invention may be used with other types of printing systems. For example, printer section 36 may instead use a different type of rendering device or print engine, such as ink jet, ionographic, lithographic, thermal and photographic, among others.
For off-site image input, image input section 32 has a network connection 38 with a suitable communication channel such as an EtherNet® connection enabling image data in the form of image signals or pixels from one or more remote sources to be input to system 30 for processing. Where the Page Description Language (PDL) of the incoming imaging data is different than the PDL used by system 30, suitable conversion means (not shown) are provided. Other remote sources of image data such as streaming tape, floppy disk, video camera, among others are within the scope of this invention.
For on-site image input, section 32 has a document scanner section 40 with a Universal Document Handler (UDH) 42 for the purpose of automatically and sequentially placing and locating sets of multiple documents for scanning. Scanner section 40 incorporates one or more linear light-sensitive arrays 44 for reciprocating scanning movement below platen 46 and focused on a line-like segment of platen 46 and the document being scanned thereon. Array 44, which may utilize Charge-Coupled Device (CCD) technology or the like, provides image elemental signals or pixels representative of the image scanned which are input to processor 48 for processing.
Processor 48 communicates with the controller section 34 and includes a scanner system control 48a, an automatic gain control printed wiring board (AGCPWB) 48b and a processor 48c. AGCPWB 48b converts the analog image signals output by array 44 to digitally represented facsimile signals, and processor 48c processes the digital image signals as required to enable controller section 34 to store and handle the image in the form and order required to carry out the job programmed. After processing, the image signals are output to controller section 34. Image signals derived from network 38 are similarly input to processor 48c. 
Processor 48c also provides enhancements and changes to the image signals such as filtering, thresholding, screening, cropping, scaling (reduction/enlargement), etc.
Printer section 36 comprises a laser-type printer having a Scan Line Buffer 49, Raster Output Scanner (ROS) 50, Print Module 52, Paper Supply 54, Finisher 56, and Printer System Control 38. ROS 50 has a two-beam laser with the beams modulated in accordance with the content of an image signal input by acousto-optic modulator to provide dual-imaging beams which are scanned across a moving photoreceptor of Print Module 52 by means of a rotating polygon. This exposes two image lines on the photoreceptor with each scan to create the latent electrostatic images represented by the image signal input to the modulator.
The latent electrostatic images are developed and transferred to a print media delivered by paper supply 54. As will be appreciated by those skilled in the art, print media can comprise a selected one of various known substrates which are capable of accepting an image, such substrates including transparencies, preprinted sheets, vellum, glassy covered stock, film or the like. The print media may comprise any of a variety of sheet sizes, types and colors, and for this, plural media supply trays are provided. The transferred image is permanently fixed or fused and the resulting prints discharged to either output tray 60 (FIG. 2), or to finisher 62. Finisher 62 provides certain finishing selections such as a stitcher for stitching or stapling the prints together to form books, a thermal binder for adhesively binding the prints into books, and/or other finishing options such as slitting, perforating, saddle stitching, folding, trimming, or the like.
Color consistency control algorithms such as those described in reference to FIG. 1 are constantly being improved upon. Additionally, alternative techniques, including for example, techniques involving gray balance control have been introduced. As a result, image processing systems such as that depicted in FIG. 2 and FIG. 3, and available alternative systems, are now capable of maintaining color within a few ΔEcmc. However, control algorithms such as those described in reference to FIG. 1 do not take into account, and therefore do not compensate for, all the variables or image attributes in an imaging system. For example, the color consistency control system of FIG. 1 does not take into account the effect of media selection has on color. Additionally, the color consistency control system of FIG. 1 does not consider the effects of an interaction of a selected halftone screen with a particular kind of media.
The effects of image attributes such as media selection and halftone screen selection were once considered insignificant. However, as color consistency control techniques have improved, the effect of such image attributes has become noticeable and problematic.
Therefore, there has been a desire for a method of compensating for system performance changes such as color changes, due to image attribute changes.